1. Field of the Invention
The present invention relates generally to an injection molding die mounted on an injection molding machine for molding a plurality of optical disc base boards or the like simultaneously. More particularly, the present invention relates to an improvement of an injection molding die of the foregoing type which ensures that a number of optical disc base boards or the like can be accurately produced without influence from the thermal expansion of the injection molding die due to the temperature difference between a die assembly on a stationary side and a die assembly on a movable side.
2. Description of the Related Art
To produce a large number of optical discs or the like on a mass production basis, there has been a need for providing an injection molding die that can simultaneously mold a plurality of products at a time. To satisfy tile foregoing need, a variety of experiments have been conducted on an actual machine basis to develop an injection molding die which ensures that a plurality of optical disc base boards are simultaneously molded. However, information on successful results obtained from these experiments was not reported until now.
When a plurality of die cavities are simultaneously used for an injection molding operation, there arises the following significant problem. The center axes of adjacent die cavities are positionally offset from each other. This is due in part to differences of thermal expansion between a die assembly on the movable side and a die assembly on the stationary side which is a result of die temperature variations caused during a series of injection molding operations. This is also due to inexact movement of a platen on the movable side mounted on an injection molding machine.
To obviate the foregoing problem, tile inventors carried out various examinations with a typical conventional injection molding die, as shown In FIG. 7 and FIG. 8, for simultaneously molding four optical disc base boards.
To facilitate understanding of the present invention, the conventional injection molding die will be described below with reference to FIG. 7 and FIG. 8.
FIG. 7 is a front view of the injection molding die, particularly showing how four die cavities are arranged oil a die assembly oil the stationary side. FIG. 8 is a sectional side view of the injection molding die shown in FIG. 7. In practice. More components than that shown In tile drawings are employed in the injection molding die, but only essential components are shown for of simplification of illustration.
In FIG. 8, reference character A designates a die assembly on the stationary side. The die assembly A includes a base plate 1 which is fixedly mounted on a stationary platen (not shown) of an injection molding machine.
A quartered hot runner block (not shown) is incorporated in tile base plate 1. A molten resin infeed hole 2 is formed along a center axis of the base plate 1 for feeding a molten resin through four molten resin flow holes 3 and into four cavities 4.
An injection hole 5 is formed along the center axis of each of the four cylindrical cavities 4. In addition, to ensure that the center axis of each cavity 4 is correctly located ill alignment with the center axis of an opponent cavity 9 on the movable side, a female cylindrical guide surface 6 is formed around an inner periphery of the cavity 4.
The respective cavities 4 are fixedly mounted on the base plate 1 by tightening a plurality of bolts 7 that are circumferentially located in an equally spaced relationship on the locus of a circle having a radius R, as measured from a center point O of the base plate 1.
In FIG. 8, reference character B designates a die assembly oil the movable side. The die assembly B includes a base plate 8 which Is fixedly mounted on a platen (not shown) of the injection molding machine on the movable side.
Four cavities 9 on the movable side are fixedly mounted on the base plate 8 by tightening a plurality of bolts 14. Each cavity 9 has a small depth, and a bottom surface 10. Each cavity 9 includes a cylindrical male guide surface 11 which fits exactly into the cylindrical female guide surface 6 of the opponent cavity 4. An ejector bush, a hole cutting punch, an ejector pin and a mechanism for actuating these components (not shown) are incorporated In a cylindrical projection 12 which is integral with each cavity 9.
The cylindrical projection 12 is inserted into a hole 13 which is formed through the base plate 8.
The aforementioned conventional injection molding die has the following problems, each of which will be described below with reference to FIG. 9 and FIG. 10.
The first problem is caused by the difference in thermal expansion, which is closely associated with temperature differences between the base plate 1 on the stationary side and the base plate 8 on the movable side.
For the convenience of description, it Is assumed that the base plate 1 has a temperature higher than that of the base plate 8.
As shown in FIG. 9, base plate 1 thermally expands as represented by dotted lines, although the illustrated thermal expansion of base plate 1 is exaggerated for easy understanding.
As base plate 1 thermally expands center points Q1 to Q4 of the cavities 4 are dislocated outward from center points P1 to P4 of the cavities 8. The direction of dislocation of the center points Q1 to Q4 relative to center points P1 to P4 is coincident with the direction of extension of two lines radially extending through the center point 0 of the base plates 1 and 8.
A second problem is attributable to the structure of an injection molding machine. This problem will be described in detail below with reference to FIG. 10.
In the drawing, reference numeral 15' designates a platen on the stationary side, and reference numeral 15 designates a machine frame. The platen 15' and the machine frame 15 are integrated with the injection molding machine.
Four guide rods 16 are fixedly secured to the platen 15' and the machine frame 15. The guide rods 16 extend through bearings 19 that are fit into a platen 17 on the movable side, so that the platen 17 is slideably displaceable in a horizontal direction along the guide rods 16 by activating a driving mechanism 18. An annular clearance .DELTA.c is formed between bearing 19 and the opponent guide rod 16.
In FIG. 10, reference character A designates a die assembly on the stationary side, and reference character B designates a die assembly on the movable side.
With this construction, when the platen 17 is driven in the horizontal direction it is simultaneously lowered in a vertical direction by its own dead weight and by the weight of the die assembly B by a distance equivalent to the annular clearance .DELTA.c. In other words, the center line of the die assembly B is positionally offset from the center line of the die assembly A by a quantity equivalent to the annular clearance .DELTA.c. Further, since deflection of the guide rods 16 adds to the foregoing downward dislocation of the platen 17, the total quantity of dislocation of the platen 17 amounts to a value of .DELTA.S.
Thus, since the center line of cylindrical male guide surface 11 is positionally offset from the center line of cylindrical female guide surface 6, an abnormally high contact pressure is localized on the cylindrical male guide surface 11. This results in unavoidable damage or injury to each cylindrical male guide surface 6.
For example, if the difference in temperature between the base plate 1 and the base plate 8 is 20.degree. C., and the locus of a circle circumferentially extending through the center points P1 to P4 has a radius R of 150 mm, the deviation of the center axis of the cavity 9 from that of the cavity 4 amounts to about 0.04 mm.
Thus, since it is estimated that the quantity of dislocation .DELTA.S of the platen 17, derived from the deformation of the injection molding machine, ranges from 0.1 to 0.3 mm, a large load is exerted on the respective cylindrical guide surfaces 6 and 11.
If, for example, an injection molding die for simultaneously molding four optical disc base boards, each having a diameter of 120 mm, is mounted on the injection molding machine, it is estimated that the total weight of the platen 17 and the die assembly B amounts to about 400 Kg. In this case, a surface pressure appearing around each of the cylindrical guide surfaces 6 and 11 is comparatively small, even though the quantity of dislocation .DELTA.S is large.
As the platen 17 is thermally expanded due to the temperature difference between the base plate 1 and the base plate 8, the platen 17 Is elastically deformed. This causes a high strain in the platen 17, even though the absolute value of dislocation of the platen 17 is very small. Consequently, a very high and intense contact pressure acts on each of the cylindrical guide surfaces 6 and 11.
As is apparent from the above description, in contrast with an injection molding die for molding a single optical disc base board, the aforementioned injection molding die for simultaneously molding a plurality of optical disc base boards can not exactly locate the center axis of each cavity in alignment with the center axes of the base plates of the injection molding die mounted oil the injection molding machine.
In summary, in contrast to the injection molding die for molding a single optical disc base board, the injection molding die for simultaneously molding a plurality of optical disc base boards has two problems to be solved: the dimensional dislocation of the base plate on the movable side from the base plate on the stationary side due to unavoidable differences of thermal expansion associated with the temperature difference between the die assembly on the movable side and the die assembly on the stationary side, and the accumulative deviation of the center axis of each die cavity on the movable side from the center axis of each die cavity on the stationary side because of positional disalignment of the center axis of the platen oil the movable side with the center axis of the platen on the stationary side.